The present invention generally relates to a method and an apparatus for transferring wafers in a process machine by using a vacuum pad and more particularly, relates to a method and apparatus for eliminating wafer breakage problem during wafer transfer by a vacuum pad in a process machine by cleaning the pad with a rotating brush or by applying vacuum pressure to the pad at a slow rate.
In semiconductor processing, the process of backside grinding, or backlapping process, is frequently used in reducing the thickness of a wafer and in correcting any curvature in the wafer to achieve planarity and parallelism of the top and bottom surfaces of a wafer. Since a wafer polishing process can only remove a maximum thickness of about 5 xcexcm, the polishing process cannot be used effectively to thin down a wafer or to correct the curvature of a wafer and achieve parallelism of the surfaces. Furthermore, the backside grinding or the backlapping process is frequently used before a wafer polishing process to achieve major thickness reductions. The wafer backside grinding process can be carried out in a lapping apparatus which may be a single-side lapping or a double-side lapping.
In a commercial backside grinding apparatus, such as one made by the Disco Company of Japan, a wafer transfer arm or a robotic arm is used for transferring wafers from one polishing station to another polishing station. For instance, in a backside grinding apparatus, there are usually at least three separate stations of a course grinding station, a fine grinding station and a cleaning/spin dry station.
As shown in FIG. 1, a T-shaped arm 12 is used in a backside grinding apparatus chamber 10 for transferring wafers from station to station. The T-shaped transfer arm 12 is constructed by a robotic arm 14 equipped with three vacuum conduits 16,18 and 20. At the end of each of the vacuum conduits 16xcx9c20, is provided with a wafer transfer pad 22. The wafer transfer pad is fabricated of a sintered ceramic material that has a multiplicity of vacuum passageways therethrough. The wafer transfer pads 22 are shaped in the shape of a flat disk that has a diameter of approximately 4xe2x80x3 when used to transfer 6xe2x80x3 wafers. As shown in FIG. 1, wafers 24 are positioned at the center of chuck tables 26 for processing at various polishing stations. The sintered ceramic material may be formed of glass powders that are fused together at a high sintering temperature forming a sponge-like structure and thereby providing a multiplicity of vacuum passageways therethrough.
The wafer transfer pad 22 functions when the T-shaped arm 12 turns at an angle, as shown in FIG. 2, such that the wafer transfer pads 22 are positioned directly on top of the wafers 24. After a vacuum is withdrawn from the conduits 18,20, the wafer 24 is picked-up by the wafer transfer pads 22. After the wafers 24 are picked-up by the wafer transfer pads 22, the T-shaped arm 12 turns to a predetermined angle such that wafers 24 can be either unloaded from the backside grinding apparatus 10 or transferred to the next process station by positioning the wafer 24 onto a chuck table 26.
During the operation of the backside grinding apparatus, various grinding debris and by-products in the form of particles are generated. In order to avoid contamination by the debris or particles to the wafer surface, the wafer transfer pads 22 must be frequently cleaned in order to remove any particle that is left on the surface of the pad 22.
A typical wafer transfer pad cleaning process and apparatus are shown in FIG. 3. The cleaning apparatus 30 consists of a solenoid valve 32 for the vacuum source, a vacuum sensor 34, a solenoid valve 36 for the water and air mixed flow, flow meters 38,40 for air and water, respectively, and a wafer transfer pad 22. A water/air mixture is fed through the backside of the wafer transfer pad 22 by conduit 42. The water/air mixture purges through a thickness of the wafer transfer pad 22 to flush out all contaminating particles and backside grinding debris.
During a normal wafer transfer process, vacuum pressure is applied through the wafer transfer pad 22 for picking-up a wafer. A profile of the vacuum pressure vs. time is shown in FIG. 4 for a conventional vacuum system. It is seen that within a period of time of 1 sec., the vacuum pressure applied increases from about 15 psi to about 75 psi. The rate of pressure increase is therefore about 60 psi/sec.
Problems are frequently caused by either the conventional pad cleaning process shown in FIG. 3, or by the conventional vacuum pressure system shown in FIG. 4. For instance, the conventional pad cleaning method of FIG. 3 is not always effective in removing all backside grinding debris and contaminating by-product particles. Any residual particles left on the surface of the wafer transfer pad 22 creates a pressure point when the particle is caught in-between the wafer transfer pad and a wafer. Such pressure point, or stress concentration point, can cause wafer breakage. The conventional vacuum pressure system of FIG. 4 may further cause problems in that the sudden increase in the vacuum pressure, i.e. at a rate of about 60 psi per sec., may impact the wafer and cause the wafer to break or fracture.
It is therefore an object of the present invention to provide a wafer transfer pad cleaning method that does not have the drawbacks or shortcomings of the conventional pad cleaning process.
It is another object of the present invention to provide a wafer transfer pad cleaning method in which all contaminating particles can be removed from the surface of the wafer transfer pad.
It is a further object of the present invention to provide a wafer transfer pad cleaning method by utilizing a rotating brush on the surface of the pad simultaneously with the spray of a cleaning solvent.
It is another further object of the present invention to provide a wafer transfer pad cleaning process that utilizes a rotating brush which rotates at a speed of less than 500 rpm.
It is still another object of the present invention to provide a vacuum pressure system in a wafer transfer pad that does not cause breakage of the wafer when contacting the wafer due to a sudden pressure surge.
It is yet another object of the present invention to provide a vacuum pressure system for a wafer transfer pad which is adapted to apply a vacuum pressure at a rate smaller than 30 psi/sec.
It is yet another further object of the present invention to provide a vacuum pressure system for a wafer transfer pad that does not cause an impact on the wafer and subsequent breakage of the wafer upon contacting the wafer during the pick-up process.
It is still another further object of the present invention to provide an apparatus for eliminating wafer breakage during wafer transfer by a vacuum pad by incorporating a pressure regulating valve situated in a vacuum conduit for regulating a vacuum pressure applied at a rate not higher than 30 psi/sec. to the surface of the wafer transfer pad.
The method can be carried out by first providing a wafer transfer pad that is fabricated of a sintered ceramic material, i.e. such as a sintered glass, with a multiplicity of vacuum passageways therethrough, then cleaning a surface of the pad that contacts a wafer by contacting a rotating brush and a spray of water, then removing substantially all particles from the surface of the pad, contacting the surface of the pad to a surface of the wafer, applying a vacuum pressure to the surface of the pad by withdrawing air through the multiplicity of vacuum passageways, and picking-up the wafer and transferring to a different process station in the grinding apparatus.
The invention further discloses an apparatus for eliminating wafer breakage during a wafer transfer process in a grinding apparatus by utilizing a wafer transfer pad. The apparatus includes a wafer transfer pad that is fabricated of a sintered ceramic material, such as a sintered glass material, with a multiplicity of vacuum passageways therethrough, a vacuum conduit for applying a vacuum pressure on a backside of the wafer transfer pad from a vacuum source, and a pressure regulating valve situated in the vacuum conduit for regulating a vacuum pressure applied at a rate not higher than 30 psi/sec., and preferably not higher than 10 psi/sec. to the surface of the wafer transfer pad.